Return trips to Mars pose unacceptable radiation risk

Bad news wannabe Mars explorers. The round trip alone would blast you with doses of radiation that come close to the acceptable lifetime limits sets by NASA – and that’s not including any practice trips to space or time spent on the Red Planet.

Charged, energetic particles like cosmic rays can wreak havoc on delicate biological tissue. Earth’s atmosphere and magnetic field serve to block or deflect most of this radiation, but astronauts on trips to Mars would be exposed to much higher doses.

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Exactly how high wasn’t clear, however. Previous measurements of the radiation in deep space came from unshielded detectors on telescopes, which overestimated how many particles would reach astronauts protected by a spacecraft.

As Curiosity prepared to take off for the Red Planet in November 2011, NASA saw an opportunity&colon; tucked inside a capsule, parts of the rover would experience a comparable amount of shielding as a crew bound for Mars. So the space agency added a monitor to Curiosity that could detect two types of space radiation for most of the 253-day, 560-million-kilometre trip to the planet.

Galactic rays

Each poses a slightly different threat. The solar cosmic rays are of relatively low energy, just a few hundred megaelectron volts (MeV). These are more easily blocked by shielding, but their numbers can spike during a solar flare or coronal mass ejection. Galactic cosmic rays, however, can reach 1000 MeV and include much heavier particles like carbon or iron ions, and are thought to pose more of a danger to people.

The Radiation Assessment Detector (RAD) mounted on Curiosity uses a mixture of silicon and plastic detectors to track the energies and directions of both types of cosmic ray. Now the RAD team has converted the data collected as Curiosity cruised towards Mars into sieverts, a measure of how a given dose of radiation affects the body.

NASA currently limits an astronaut’s acceptable radiation dose to a 3 per cent risk of exposure-induced death from cancer over a lifetime. That amounts to 0.6 to 1 sieverts for women and 0.8 to 1.2 sieverts for men, assuming these individuals have never smoked and are between 30 and 60 years old.

The RAD results show that a crew on a 180-day journey to Mars would receive 0.331 sieverts, which the return trip would bring up to 0.662. This is just over the lower limit for women, and close to the lower end of the limit for men. Taken with any previous exposure the same voyagers might have had prior to the trip, such as during trial runs to the International Space Station, and any time actually spent on the surface of Mars, this could amount to an unacceptable risk.

“The numbers we report are on the high side,” says principal scientist Cary Zeitlin of the Southwest Research Institute in Boulder, Colorado. Adding more shielding to the spacecraft would be one way to reduce the dose – though the extra weight would create its own challenges. Another way to reduce the risk would be to speed up the journey. “The best thing would be to get there faster,” says Zeitlin.

Cellular billiards

Because of its thin atmosphere, we know that the planet’s surface receives less shielding from radiation than Earth – though how much less is unclear. RAD has continued to measure radiation levels during Curiosity’s ground mission, and Zeitlin hopes to report those results soon.

Last year, indirect evidence – also from Curiosity – suggested that radiation levels on Mars’s surface were about the same as those in the ISS – so higher than on Earth but not astronomical.

But some still think the surface might pose even more of a danger than the journey to get there. That’s because radiation from space can strike particles in the rocks and ice, creating a secondary shower of neutrons that would hit human tissue. The neutrons could smack into protons inside human cells and knock them around like billiard balls, doing damage to DNA that is difficult for the cell to repair, says Lewis Dartnell of the University of Leicester, UK, who was not involved in the new research.

“We don’t truly understand the biological effects of this radiation,” Dartnell says. “Getting readings on the Martian surface are critical for understanding the human risk there, and how to design shielding or countermeasures for protecting them.”

That would be bad news even for proposed one-way missions, such as Mars One, which is currently accepting astronaut applications.